The present invention relates to crosslinking agents based on modified aminoplast resins and to powder coating compositions containing these crosslinking agents.
In recent years, powder coatings have become increasingly popular because these coatings are inherently low in volatile organic content (xe2x80x9cVOCxe2x80x9d), which significantly reduces emissions of volatile organic compounds into the atmosphere during application and curing processes.
Hydroxyl, carboxyl, carbamate, and/or epoxy functional resins, such as acrylic and polyester resins having relatively high glass transition temperatures (xe2x80x9cTgxe2x80x9d), are commonly used as main film-forming polymers for these coatings. The relatively high Tg of such polymers provides powder coatings having good storage stability. However when exposed to the extreme temperatures which can be encountered in many geographic areas during shipping and/or storage, even better powder coating stability is desired. By xe2x80x9cstorage stabilityxe2x80x9d is meant the ability of the individual powder particles which comprise the powder coating to resist the tendency to adhere to one another, thereby causing xe2x80x9cclumpingxe2x80x9d or xe2x80x9cfusingxe2x80x9d of the powder coating composition upon storage prior to application. Powder coating compositions having very poor storage stability can be difficult, if not impossible, to apply.
Aminoplast resins are well known in the art as low cost crosslinking agents for hydroxyl, carboxyl, and/or carbamate functional polymers in conventional liquid coating compositions. Common aminoplast resins are based on condensation products of formaldehyde with an amino- or amido-group carrying substance. Examples of these aminoplast resins include the methylol and alkoxymethyl derivatives of ureas, melamines, and benzoguanamines which are most commonly used in liquid coating compositions. Such aminoplast resins can provide enhanced coating properties such as exterior durability, chemical resistance, and mar resistance.
Attempts to produce powder coating compositions based on conventional aminoplast resins which exhibit these desirable properties heretofore have been unsatisfactory because these materials typically are in liquid form and, as such, can cause poor powder stability.
The methoxylated aldehyde condensates of glycoluril, which are solid products, are the aminoplast resins most commonly employed as crosslinking agents in powder coating compositions. Although solid in form, these materials nonetheless can depress the Tg of the powder coating composition significantly, even when combined with high Tg film-forming polymers such as the acrylic polymers described above. Such a depression in Tg also can result in poor powder stability.
Moreover, the use of conventional aminoplast resins in powder coating compositions can result in the phenomenon commonly referred to as xe2x80x9cgassingxe2x80x9d. Gassing can occur as a result of vaporization of the alcohol generated in the thermally induced aminoplast crosslinking reaction. The alcohol vapor is driven off through the coating film upon heating and, as the viscosity of the coating increases during the curing process, xe2x80x9cpinholesxe2x80x9d or xe2x80x9ccratersxe2x80x9d are formed as the gas escapes through coating surface.
U.S. Pat. No. 3,759,854 discloses heat-fusible powder coating compositions prepared by pre-reacting a thermosetting polyester resin and a suitable crosslinking resin such as a condensation product of an aldehyde with melamine, urea or benzoguanamine. The reaction product typically is prepared from 1 to 50 weight percent of the amine-aldehyde material and the reaction is carried to an extent such that the resulting powder has a softening point of at least 65xc2x0 C.
U.S. Pat. No. 5,302,462 discloses a process for preparing a partially cured, but non-gelled, powder coating composition. The powder coating composition is prepared by reacting a less than stoichiometric amount of methoxymethyl aminotriazine with a substantially linear, hydroxyl-terminated polyester. The ratio of polyester to triazine ranges from 97:3 to 70:30 by weight. Methanol is stripped from the reaction mixture under reduced pressure. The powder coating composition provides a cured film free of surface defects caused by outgassing of methanol which volatilizes during the curing process.
U.S. Pat. No. 3,980,732 discloses a process for preparing a curable powder resin composition having a sintering temperature above 40xc2x0 C. The method comprises condensing a methylolamino compound with an aliphatic alcohol and an aliphatic diamide to produce an aminoplast condensate with a Tg ranging from xe2x88x9210xc2x0 C. to 100xc2x0 C. and blending the aminoplast condensate with an acrylic or polyester resin having a glass transition temperature ranging from 60xc2x0 C. to 100xc2x0 C. The methylolamino compound is selected from methylolureas and methylolaminotriazines and contains no more than one unmethylolated NH bond per molecule. At least half of the methylol groups of the methylolamino compound have been condensed with the aliphatic alcohol or the aliphatic diamide. The molar ratio of the aliphatic alcohol to the methylolamino compound is at least 2; and the molar ratio of the methylolamino compound to the aliphatic diamide ranges from 1:0.5 to 1:1.5.
U.S. Pat. No. 4,185,045 discloses a powder coating composition comprising a solid crosslinking agent having a softening point ranging from 50xc2x0 C. to 120xc2x0 C. and prepared by heating 40 to 75% by weight of an acrylic polyol and 60 to 25% by weight of an alkoxyaminotriazine at 50xc2x0 to 120xc2x0, and a base resin having a softening point ranging from 60xc2x0 C. to 130xc2x0 C. The alkoxyaminotriazine has less than 1 non-methylolated NH bond per triazine ring and at least 80% of the methylol groups have been etherified with an aliphatic or alicyclic alcohol or ethylene glycol monoalkyl ether.
U.S. Pat. No. 4,230,829 discloses a solid crosslinking agent having a softening point of 50xc2x0 C. to 120xc2x0 C. and prepared by heating 40 to 70% by weight of a polyester polyol and 60 to 30% by weight of an alkoxyaminotriazine. The alkoxyaminotriazine has one or less non-methylolated NH bond per triazine ring and at least 80% of the methylol groups have been alkoxylated with an alcohol. Powder coating compositions are prepared from a mixture of the crosslinking agent and a base resin having a softening point of 60xc2x0 C. to 130xc2x0 C.
While the above-described prior art aminoplast-based crosslinkers for powder coating compositions provide some improvement in xe2x80x9cgassingxe2x80x9d and powder stability over their liquid aminoplast counterparts, the powder coating compositions containing these crosslinkers can, nonetheless, exhibit some of the aforementioned deficiencies. In addition, many of the crosslinkers disclosed in the prior art can be high molecular weight, high viscosity, partially cured, and, thereby, unstable mixtures. Thus, there remains a need for an aminoplast crosslinking agent suitable for use in powder coating compositions which provides a storage stable powder composition having the desirable coating properties usually associated with aminoplast-based liquid coatings without causing coating surface defects due to xe2x80x9cgassingxe2x80x9d.
In accordance with the present invention, provided is a crosslinking agent comprising the ungelled reaction product of (a) at least one aminoplast resin; and (b) at least one compound having active hydrogen groups reactive with aminoplast resin (a). The compound (b) is selected from at least one of (i) compounds having the following structure (I): 
wherein X is aromatic; R1, R2, and R3 can be the same or different and each independently represents H, (cyclo)alkyl having from 1 to 12 carbon atoms, aryl, alkaryl, aralkyl, or an active hydrogen-containing group, provided that at least one of R1, R2, and R3 represents an active hydrogen-containing group which is reactive with the aminoplast resin (a); (ii) compounds having the following structure (II or III): 
where Rxe2x80x2 and Rxe2x80x3 are the same or different and each independently represents an aromatic group or an alkyl group having 1 to 12 carbon atoms; and (iii) compounds different from both (i) and (ii) and having a melting point of at least 80xc2x0 C. The crosslinking agent has a glass transition temperature of at least 10xc2x0 C. and is essentially free of functional groups which are reactive with aminoplast resin.
Also provided is a method for preparing the aforementioned crosslinking agent. The method comprises the steps of (I) combining (a) at least one aminoplast resin and (b) at least one compound having active hydrogen groups reactive with aminoplast resin (a). The compound (b) is selected from (i) compounds having the previously described structure (I) where X, R1, R2, and R3 are as described above for that structure, and (ii) compounds having the structures (II) and (III) as described above; and (iii) compounds different from (i) and (ii), and having a melting point of at least 80xc2x0 C., to form a reaction admixture; (II) heating the reaction admixture to a temperature ranging from 90xc2x0 C. to 135xc2x0 C.; and (III) maintaining the temperature achieved in (II) for a time sufficient to obtain an ungelled reaction product having a glass transition temperature of at least 10xc2x0 C. which is essentially free of active hydrogen-containing groups as determined by infrared spectroscopy.
Further provided is a curable powder coating composition comprising a solid particulate film-forming mixture of (A) a polymer containing reactive functional groups and having a Tg of at least 30xc2x0 C. and (B) the crosslinking agent described above.
Also provided is a multi-layer composite coating composition comprising a base coat deposited from a film forming composition and a top coat over at least a portion of the base coat. The top coat is deposited from the aforedescribed curable powder coating composition. Coated substrates are also provided.